Uncovering the kinetics of Li-rich clusters and monodisperse core–shell Al₃(Zr, Sc) structures in Al–Li–Cu alloys

The size distribution and structural evolution of the precipitates are critical to the mechanical properties of heat treatable alloys. In Al alloys, many studies have reported that monodisperse core–shell Al₃Li/Al₃(Zr, Sc) structures in a Li-rich environment promote formation of δ′-Al₃Li, T₁-Al₂CuLi, and θ′-Al₂Cu precipitates. However, without quantitative measurements of the precipitate size distribution and tracking their crystal structure evolution, the kinetics of Li-rich clusters and excessive Li diffusion into monodisperse core–shell Al₃(Zr, Sc) structures in Al alloys to prevent Li-cluster coarsening is still unknown. In this work, in-situ small angle neutron scattering (SANS) has been adopted to track the evolution of δ′-Al₃Li precipitates. Upon quenching, Li-rich clusters remain at a smaller size and higher number density. Once aging starts, nucleation of δ′-Al₃Li precipitates on top of Al₃(Zr, Sc) is at the expense of dissolution of those Li-rich clusters below the critical size. From density functional theory (DFT) calculations, it has been found the δ′-Al₃Li precipitate nucleation barrier on top of Al₃(Zr, Sc) is negligible, as well as the nucleation barrier of θ′-Al₂Cu on top of Al₃(Zr, Sc) is much lower relative to that on α-Al. Besides, the substitutional energy for Li to replace Zr and Sc sites in Al₃(Zr, Sc) is favorable. Combining SANS with high-resolution transmission electron microscopy (HRTEM), the effects of Al₃(Zr, Sc) cores on preventing δ′-Al₃Li size from coarsening by absorbing Li to form Al₃(Li, Zr, Sc) have been uncovered. Simultaneously, the contributions of Al₃(Li, Sc, Zr) particles and δ′-Al₃Li to the strength has been quantified. This work opens a new engineering approach of precise control of precipitating kinetics at the presence of monodisperse precursors for industrially relevant structural materials.